Heretofore, data has been electromagnetically recorded along data tracks on a rotating disk. Optically monitorable structures or pits were arranged along the disk to refine the position of the tracks. More specifically, the tracks were arranged in concentric circles or in a spiral around an axis of rotation of the disk. Conventionally, the pits were disposed to either side of the track. A concentrated laser lightspot was passed along the track, between the pits, and the intensity and polarity of reflective light were monitored. As the laser passed over a pit, the intensity of reflected light was changed.
In perfect alignment, the reflected light intensity reduction as a spot passed a pit on one side of the track was the same as the reflected light intensity reduction when the spot passed a pit on the other side of the track. When the pits on one side of the track reduced the intensity of the reflected light more than pits on the other side, a misalignment was indicated. By monitoring the difference in the intensity of light reflected as a spot passed the pits on the left side of the track relative to the intensity of reflected light as the spot passed pits on the right side of the track, the tracking error could be determined. See for example U.S. Pat. No. 4,562,564 to Bircot et al., issued Dec. 31, 1985. A servo system adjusted the position of the lightspot and any associated reading structure or readhead relative to the rotating disk in accordance with the monitored left-right pit intensity difference.
Although laser light tracking systems were effective for maintaining tracking alignment, small tracking errors were inherent in the servo control and correction system. Even small tracking errors as small as 0.1 micrometers caused significant variations in the amplitude of the read data signal. As the amplitude was reduced, interpretation of the data signal became ambiguous. For example, when reading digital data, the amplitude of the analog value representing 1's was reduced. With sufficient misalignment, the analog amplitude of a digital 1 was reduced into the amplitude region allocated for 0's causing an encoded 1 to be read as a 0. Reading digital 1's as 0's, of course, caused significant errors in the retrieved data.
In accordance with the present invention, a method and apparatus is provided for reducing ambiguities in the retrieved data attributable to readhead/track misalignment.